One alcohol of great interest today is ethanol, which can be produced from virtually any type of grain, but is most often made from corn. Most of the fuel ethanol in the United States is produced from a wet mill process or a dry grind ethanol process. Although virtually any type and quality of grain can be used to produce ethanol, the feedstock for these processes is typically a corn known as “No. 2 Yellow Dent Corn.” The “No. 2” refers to a quality of corn having certain characteristics as defined by the National Grain Inspection Association, as is known in the art. “Yellow Dent” refers to a specific type of corn as is known in the art.
The conventional methods for producing various types of alcohol from grain generally follow similar procedures, depending on whether the process is operated wet or dry. Wet mill corn processing plants convert corn grain into several different co-products, such as germ (for oil extraction), gluten feed (high fiber animal feed), gluten meal (high protein animal feed), and starch-based products such as ethanol, high fructose corn syrup, or food and industrial starch. Dry grind ethanol plants convert corn into two products, namely ethanol and distiller's grains with solubles. If sold as wet animal feed, distiller's wet grains with solubles is referred to as DWGS. If dried for animal feed, distiller's dried grains with solubles is referred to as DDGS. In the standard dry grind ethanol process, one bushel of corn yields approximately 8.2 kg (approximately 17 lbs) of DDGS in addition to the approximately 10.5 liters (approximately 2.8 gal) of ethanol. This co-product provides a critical secondary revenue stream that offsets a portion of the overall ethanol production cost.
With respect to the wet mill process, FIG. 1 is a flow diagram of a typical wet mill ethanol production process 10. The process 10 begins with a steeping step 12 in which corn is soaked for 24 to 48 hours in a solution of water and sulfur dioxide in order to soften the kernels for grinding, leach soluble components into the steep water, and loosen the protein matrix with the endosperm. Corn kernels contain mainly starch, fiber, protein, and oil. The mixture of steeped corn and water is then fed to a degermination mill step (first grinding) 14 in which the corn is ground in a manner that tears open the kernels and releases the germ so as to make a heavy density (8.5 to 9.5 Be) slurry of the ground components, primarily a starch slurry. This is followed by a germ separation step 16 that occurs by flotation and use of a hydrocyclone(s) to separate the germ from the rest of the slurry. The germ is the part of the kernel that contains the oil found in corn. The separated germ stream, which contains some portion of the starch, protein, and fiber, goes to germ washing to remove starch and protein, and then to a dryer to produce about 2.7 to 3.2 Lb. (dry basis) of germ per bushel of corn. The dry germ has about 50% oil content on a dry basis.
The remaining slurry, which is now devoid of germ, but containing fiber, gluten (i.e., protein), and starch, is then subjected to a fine grinding step (second grinding) 20 in which there is total disruption of endosperm and release of endosperm components, namely gluten and starch, from the fiber. This is followed by a fiber separation step 22 in which the slurry is passed through a series of screens in order to separate the fiber from starch and gluten, and to wash the fiber clean of gluten and starch. The fiber separation stage 22 typically employs static pressure screens or rotating paddles mounted in a cylindrical screen (Paddle Screens). Even after washing, the fiber from a typical wet grind mill contains 15 to 20% starch. This starch is sold with the fiber as animal feed. The remaining slurry, which is now devoid of fiber, is subjected to a gluten separation step 24 in which centrifugation or hydrocyclones separate starch from the gluten. The gluten stream goes to a vacuum filter and dryer to produce gluten (protein) meal.
The resulting purified starch co-product then undergoes a jet cooking step 26 to start the process of converting the starch to sugar. Jet cooking refers to a cooking process performed at elevated temperatures and pressures, although the specific temperatures and pressures can vary widely. Typically, jet cooking occurs at a temperature of about 120 to 150° C. (about 248 to 302° F.) and a pressure of about 8.4 to 10.5 kg/cm2 (about 120 to 150 lbs/in2), although the temperature can be as low as about 104 to 107° C. (about 220 to 225° F.) when pressures of about 8.4 kg/cm2 (about 120 lbs/in2) are used. This is followed by liquefaction 28, saccharification 30, fermentation 32, yeast recycling 34 and distillation/dehydration 36. Liquefaction occurs as the mixture, or “mash” is held at 90 to 95° C. in order for alpha-amylase to hydrolyze the gelatinized starch into maltodextrins and oligosaccharides (chains of glucose sugar molecules) to produce a liquefied mash or slurry. In the saccharification step 30, the liquefied mash is cooled to about 50° C. and a commercial enzyme known as gluco-amylase is added. The gluco-amylase hydrolyzes the maltodextrins and short-chained oligosaccharides into single glucose sugar molecules to produce a liquefied mash. In the fermentation step 32, a common strain of yeast (Saccharomyces cerevisae) is added to metabolize the glucose sugars into ethanol and CO2.
Upon completion, the fermentation mash (“beer”) will contain about 17% to 18% ethanol (volume/volume basis), plus soluble and insoluble solids from all the remaining grain components. The solids and some liquid remaining after fermentation go to an evaporation stage where yeast can be recovered as a byproduct. Yeast can optionally be recycled in a yeast recycling step 34. In some instances, the CO2 is recovered and sold as a commodity product. Subsequent to the fermentation step 32 is the distillation and dehydration step 36 in which the beer is pumped into distillation columns where it is boiled to vaporize the ethanol. The ethanol vapor is condensed in the distillation columns, and liquid alcohol (in this instance, ethanol) exits the top of the distillation columns at about 95% purity (190 proof). The 190 proof ethanol then goes through a molecular sieve dehydration column, which removes the remaining residual water from the ethanol, to yield a final product of essentially 100% ethanol (199.5 proof). This anhydrous ethanol is now ready to be used for motor fuel purposes.
No centrifugation step is necessary at the end of the wet mill ethanol production process 10 as the germ, fiber and gluten have already been removed in the previous separation steps 16, 22 and 24. The “stillage” produced after distillation and dehydration 36 in the wet mill process 10 is often referred to as “whole stillage” although it also is technically not the same type of whole stillage produced with the dry grind process described in FIG. 2 below, since no insoluble solids are present. Other wet mill producers may refer to this type of stillage as “thin” stillage.
The wet grind process 10 can produce a high quality starch product for conversion to alcohol, as well as separate streams of germ, fiber and protein, which can be sold as by-products to generate additional revenue streams. However, the overall yields for various by-products can be less than desirable; and the wet grind process is complicated and costly, requiring high capital investment as well as high-energy costs for operation.
Because the capital cost of wet grind mills can be so prohibitive, some alcohol plants prefer to use a simpler dry grind process. FIG. 2 is a flow diagram of a typical dry grind ethanol production process 100. The process 100 begins with a milling step 102 in which dried whole corn kernels are passed through hammer mills in order to grind them into meal or a fine powder. The ground meal is mixed with water to create a slurry, and a commercial enzyme called alpha-amylase is added (not shown). This slurry is then heated to approximately 120° C. for about 0.5 to three (3) minutes in a pressurized jet cooking process 104 in order to gelatinize (solubilize) the starch in the ground meal. It is noted that some processes exclude a jet cooker and instead have a longer hold time of the slurry in a slurry tank at a temperature from about 50° C. to 95° C.
This is followed by a liquefaction step 106 at which point additional alpha-amylase may be added. The stream after this liquefaction step has about 30% dry solids (DS) content with all the components contained in the corn kernels, including sugars, protein, fiber, starch, germ, oil and salts. This is followed by separate saccharification and fermentation steps, 108 and 110, respectively, although in most commercial dry grind ethanol processes, saccharification and fermentation occur simultaneously. This step is referred to in the industry as “Simultaneous Saccharification and Fermentation” (SSF). Both saccharification and SSF can take as long as about 50 to 60 hours. Fermentation converts the sugar to alcohol. Yeast can optionally be recycled in a yeast recycling step 112. Subsequent to the fermentation step 110 is a distillation and dehydration step 114, much like that in the wet mill process, to recover the alcohol.
Finally, a centrifugation step 116 involves centrifuging the residuals produced with the distillation and dehydration step 114, i.e., “whole stillage” in order to separate the insoluble solids (“wet cake”) from the liquid (“thin stillage”). The liquid from the centrifuge contains about 8% to 10% DS. The thin stillage enters evaporators in an evaporation step 118 in order to boil away moisture, leaving a thick syrup which contains the soluble (dissolved) solids from the fermentation (25 to 35% dry solids). The concentrated slurry can be sent to a centrifuge to separate the oil from the syrup. The oil can be sold as a separate high value product. The oil yield is normally about 0.5 Lb/Bu of corn with high free fatty acids content. The free fatty acids are created when the oil is held in the fermenter for approximately 50 hours. The free fatty acids content reduces the value of the oil. The de-oil centrifuge only removes less than 50% because the protein and oil make an emulsion, which cannot be satisfactorily separated.
The syrup, which has more than 10% oil, can be mixed with the centrifuged wet cake, and the mixture may be sold to beef and dairy feedlots as Distillers Wet Grain with Solubles (DWGS). Alternatively, the wet cake and concentrated syrup mixture may be dried in a drying step 120 and sold as Distillers Dried Grain with Solubles (DDGS) to dairy and beef feedlots. This DDGS has all the protein and 75% of the oil in corn. But the value of DDGS is low due to the high percentage of fiber, and in some cases the oil is a hindrance to animal digestion.
Because the dry mill process 100 only produces ethanol and low value DDGS, many companies have started to develop a dry fraction process. In this process, corn goes through a pretreatment step, such as steam treatment, then various types of mechanical separation equipment are utilized to separate the dry fractions of the corn, including the fiber, starch, and oil/germ portion. While these separation processes accomplish some separation of the components, the separation is generally incomplete. For example, the fiber portion normally contains more than 30% starch on a dry basis, and the germ contains more than 25% starch and 35% oil content on a dry basis. In addition, less than 30% of the total oil in the corn kernels is recovered with these processes; and the germ and fiber portions must go through purification stages before they can be sold for a reasonable price.
After the dry fractionation, the starch (with protein) goes through another grind step, then liquefaction, fermentation, distillation, and evaporation to produce alcohol and syrup, much the same as in the dry grind process 100. But the alcohol yield normally is as low as 2.3 gal/Bu of corn because of the loss of starch to the germ and fiber portions. In addition, the purification steps mentioned above for the germ and fiber are complicated and costly. Notably, the dry fraction process does not give sharp separation and produces low purity by-products, which complicates the downstream purification steps. Because of the high costs and low yields, these dry fractionation processes have not been generally accepted by the industry.
Other attempts have been made in the dry grinding industry to desirably recover high value by-products, such as oil. However, attempts to separate oil from the “hammer milled” slurry have failed because of the high concentration of solids and because the oil is not released from the solid particles. Some success has been realized with processes recovering oil from the evaporation stages of the dry mill process. However, the yield is relatively low, and the oil must move through the entire process, including fermentation, prior to evaporation. The presence of the oil in these steps of the process can be detrimental to the efficiency of the remaining parts of the process. Attempts have been made to recover the oil directly after fermentation. However, the process of mixing and fermentation emulsifies the oil, and this makes it very difficult to remove. Other attempts have been made to recover oil directly from corn by solvent extraction but the cost, for example, is too high for commercial use.
It would thus be beneficial to provide an improved system and method for separating by-products from grains used for alcohol production that overcomes various aforementioned drawbacks, such as to produce high value by-products with desirable yield.